Solid-state electrically controlled light emitters are widely used in the display and lighting industries. Displays often use differently colored emitters, and lighting applications require a large color rendering index (CRI). In either case, the efficient production of a variety of colors is important.
Colored light is produced in liquid crystal displays (LCDs) and some organic light-emitting diode (OLED) displays using white-light emitters (such as a backlight) and color filters, for example as taught in U.S. Pat. No. 6,392,340. However, this approach has the disadvantage of wasting much of the white light produced by the back light. In a different approach, light emitters emit a specific desired color. For example, some OLED displays use different organic materials to emit light of different colors. This design requires the patterned distribution of the different organic materials over a display substrate at a micron-level resolution, for example using evaporation through a mechanical shadow mask. However, it is difficult to maintain pattern accuracy using metal shadow masks over large display substrates, for example greater than 300 mm by 400 mm, or display substrates requiring high resolution.
Inorganic light-emitting diodes (LEDs) based on crystalline semiconductor materials are also used to emit light of different frequencies. These crystalline-based inorganic LEDs provide high brightness, excellent color saturation, long lifetimes, good environmental stability, and do not require expensive encapsulation for device operation, especially in comparison to OLEDs. However, the crystalline semiconductor layers also have a number of disadvantages. For example, crystalline-based inorganic LEDs have high manufacturing costs, difficulty in combining multi-color output from the same chip, low efficiency, color variability and poor electrical current response.
Most solid-state lighting products desirably emit white light with a large color rendering index, for example greater than 80 or even 90. Since solid-state light emitters emit colored rather than white light, multiple different colored solid-state light emitters are often used to provide the appearance of white light. For example, LED backlights in LCDs, or white OLED emitters in some OLED displays, use a combination of blue and yellow emitters or red, green, and blue emitters that together are experienced as white light by the human visual system. However, this technique requires the use of different light emitters that emit different colors of light. As noted above, different light emitters have different electrical and colorimetric properties leading to inefficiency and non-uniformity.
Another technique used to provide colored light is color conversion, in which a single kind of light emitter is used to optically stimulate (pump) a second light emitter with light having a first energy (frequency). The second light emitter absorbs the first light and then emits second light having a lower energy (frequency). By choosing a variety of different second light emitters that emit light of different frequencies, a display or a solid-state light device can emit light of different colors. For example, a blue light emitter can be used to emit blue light and to optically pump yellow, red, or green light emitters. U.S. Pat. No. 7,990,058 describes an OLED device with a color-conversion material layer.
Phosphors are often used as color-conversion materials. For example, U.S. Pat. No. 8,450,927 describes an LED lamp using a phosphor and U.S. Pat. No. 7,969,085 discloses a color-change material layer that converts light of a second frequency range higher than a first frequency range to light of the first frequency range. Light-emissive inorganic core/shell nano-particles (quantum dots or QDs) are also used to produce optically pumped or electrically stimulated colored light, for example as taught in U.S. Pat. No. 7,919,342.
In general, color-change material systems suffer from efficiency problems. For example, the production of relatively higher-frequency blue light can be difficult and the conversion of light from relatively higher frequencies to relatively lower frequencies may not be efficient. Moreover, the conversion materials may fade over time, reducing the performance of the display. Furthermore, much of the relatively higher-frequency light may not interact with the color-change materials and thus may not be converted to the desired, relatively lower frequency light. U.S. Patent Publication 2005/0140275A1 describes the use of red, green, and blue conversion layers for converting white light into three primary colors of red, green, and blue light. However, the efficiency of emitted-light conversion remains a problem.
Diode-pumped solid-state lasers use a solid gain medium such as a neodymium-doped yttrium aluminum garnet crystal. Light from one or more light-emitting diodes is imaged with a lens or optical fiber onto one or more sides of the crystal. The crystal then lases to produce coherent light.
Inorganic displays use arrays of inorganic light emitters, typically light-emitting diodes (LEDs). Because of the variability in LED materials and manufacturing processes, different LEDs, even when made in similar materials, will have different performances and losses in the circuits providing power to the different LEDs. LEDs made in different materials have even greater inefficiencies when provided with a common power source. Furthermore, these issues are exacerbated in LEDs since the variability of materials in a source semiconductor wafer is much greater on a smaller scale than on a larger scale. These differences lead to performance inefficiency and uniformity variations.
Although a variety of devices produce arrays of emitters emitting different colors of light, there remains a need for structures and methods that improve power efficiency and performance uniformity in the production of colored light in a simple and robust structure made with fewer parts.